The present invention relates to a technical field of a developing device used in image forming apparatus such as electrophotographic copiers and printers and, more particularly, to a technical field of a developing device comprising an image developing roll for carrying developer and conveying it to a latent image carrier.
In conventional image forming apparatus such as electrophotographic copiers and printers, an image is obtained by developing an electrostatic latent image on a latent image carrier such as photoreceptors by means of developer of a developing device, transferring the developed image from the latent image carrier to a recording medium such as paper, and finally fixing the transferred image to the recording medium.
Among conventional developing devices used in such image forming apparatus, there is a type of developing device which employs a contact development system in which an image developing roll carrying a developer and conveying it to a latent image carrier is in contact with the latter. In a developing device employing the contact development system, the image developing roll in contact with the latent image carrier conveys charged monocomponent developer to the latent image carrier, to develop the electrostatic latent image on the latent image carrier with the monocomponent developer. There are other type of conventional developing device which at least comprises an image developing roll for carrying developer and conveying it to a latent image carrier, and a regulating member which is in contact with the image developing roll and regulates the developer being conveyed to the latent image carrier in such a manner that a thin layer of developer is formed on the image developing roll surface. In such a developing device, electrically charged monocomponent developer which is regulated by the regulating member to form a thin layer of the developer on the image developing roll surface is conveyed by the image developing roll to the latent image carrier, thus developing with the thin-layered monocomponent developer an electrostatic latent image on the latent image carrier.
In this respect, a metallic roll made of aluminum or iron is used as image developing roll. Particularly, aluminum roll is commonly used because of its excellent workability and low cost.
The functions required to an image developing roll of a developing device are: (1) property for conveying developer, (2) property for charging developer, and (3) property for preventing the discharging of developing bias.
In order to improve the properties for conveying and charging of developer, a carrier roll (i.e. an image developing roll) is proposed in Japanese Patent Publication No. H6-46331, in which a metallic roll is subjected to a sand blast treatment so that a rough surface morphology is imparted to the roll surface, which is then plated with metal such as nickel. According to the carrier roll disclosed in the publication, the rough surface morphology formed on the carrier roll mechanically increases the developer conveying property of the roll, while also improving the developer charging property of the roll because of increased contact area for developer. By metal plating the rough surface of the metallic roll, the abrasion resistance of the roll is improved.
In order to prevent the discharging of the developing bias, a roll having a specific resistance set at a predetermined value has been proposed. For instance, proposed in Japanese Patent Publication No. H2-26226 is a carrier for a non-magnetic monocomponent toner (that is, an image developing roll) comprising a cylindrical rigid body made of a resin having conductive powder dispersed therein with a specific resistance value in a range of 104 to 1012 W cm in which the cylindrical rigid body has a conductive layer formed on an inner surface thereof or a conductive paint coating with a specific resistance value not exceeding 107 W cm formed on the inner surface thereof. Further, proposed in Japanese Patent No. 2705090 is a carrier for a non-magnetic monocomponent toner (that is, an image developing roll) comprising a semiconductive layer formed on its surface, the layer being formed of ceramics such as alumina, having a specific resistance value in a range of 104 to 1012 W cm and a thickness of 100 to 1000 mm. By using the carriers for non-magnetic monocomponent toner disclosed in the above mentioned publications, the discharging of the developing bias can be effectively prevented to prevent the occurrence of image defects, owing to the fact that at least the roll surface has a predetermined specific resistance value.
Meanwhile, as disclosed in above described Japanese Patent Publication No. H2-26226 and Japanese Patent No. 2705090, it is a conventional practice to use a direct current voltage overlapped with an alternate current voltage as a developing bias in a developing device in order to prevent background toning, to obtain an adequate edge effect, and to improve the gradation.
However, in the carrier roll disclosed in Japanese Patent Publication No. H6-46331, a metal plating is applied onto the roll surface which has been roughened by sand blast treatment. The metal plating comes to blur the clear morphology of the roll surface formed by sand blast treatment, in such a manner that sharp ridges bounding the adjacent recesses are rounded by overlaid plating material. Therefore, there is a problem that the effect of the rough surface morphology formed by sand blast treatment to improve the conveying property and charging property of the roll cannot be fully and reliably obtained because of the blurring of the clear surface morphology by the subsequent metal plating, although the metal plating increases the abrasion resistance of the roll surface.
Further, the carrier roll formed of a resin having conductive powder dispersed therein, disclosed in Japanese Patent Publication No. H2-26226, has a problem that it can yield mottling in the developed image because of possible uneven distribution of the conductive powder dispersed in the resin, which leads to uneven specific resistance value over the roll surface.
Furthermore, the toner carrier roll having ceramic semiconductive layer with a thickness of 100 to 1000 mm has a problem of manufacturing difficulty and cost, since the semiconductive layer is formed by spraying ceramic material melted by arc discharging, onto the toner carrier substrate.
It is conceivable to combine the technical items described in foregoing publications so that the roll can be reliably provided with the foregoing three functions. However, such combinations of technical items described in the publications for imparting the foregoing three functions to the roll can bring about following problems.
Namely, a combination of techniques of Japanese Patent Publications No. H6-46331 and No. H2-26226 means that the carrier roll is not a metallic one, but is formed of resin having conductive powder dispersed therein. There is a problem that such a carrier roll is difficult to sand blast to form a rough surface as described in Japanese Patent Publication No. H6-46331, and it is also difficult to improve its abrasion resistance by some treatment. Therefore, the combination of techniques of Japanese Patent Publications No. H6-46331 and No. H2-26226 in such a manner that the image developing roll can perform the foregoing three functions reliably is extremely difficult.
A combination of Japanese Patent Publication No. H6-46331 and Japanese Patent No. 2705090 means that a layer of ceramic semiconductive material melted by arc discharging according to Japanese Patent No. 2705090 is formed on a rough surface formed according to Japanese Patent Publication No. H6-46331. This will lead to a blurred surface morphology in such a manner that sharp ridges bounding adjacent recesses are rounded by overlaid material, as is the case in metal plating of the rough surface described in Japanese Patent Publication No. H6-46331. Therefore, the combination of techniques of Japanese Patent Publications No. H6-46331 and No. 2705090 in such a manner that the image developing roll can perform the foregoing three functions reliably is extremely difficult.
Further, the developing devices described in the above described publications employ non-contact development system which cannot be applied as it is to the contact development system, in which an image developing roll is in direct contact with a latent image carrier.
Furthermore, in all development devices described in above described three publications, a regulating member for forming a thin toner layer is in contact with an image developing roll. This type of developing device has a problem that a part of toner which is not conveyed to the latent image carrier because of regulation by the regulating member adheres to the image developing roll surface, which leads to the occurrence of a filming.
The present invention has been achieved in the light of the above described problems, and it is an object of the present invention to provide a developing device having an image developing roll capable of performing more reliably the above described three functions of conveying developer, charging developer, and preventing discharging of developing bias, and also to provide a developing device of which image developing roll can be manufactured easily at a low cost.
It is another object of the present invention to provide a developing device having an image developing roll which can prevent the filming, and which can be manufactured easily at a low cost.
It is further object of the present invention to provide a developing device in which foregoing problems can be solved by anodizing of an image developing roll, while omitting or facilitating a special treatment for sealing minute holes formed in anodizing of the image developing roll.
In order to achieve these objects, the present invention provides a developing device comprising at least: an image developing roll which can carry developer on its peripheral surface and can convey it to a latent image carrier; and a regulating member which can regulate the developer to be conveyed to said latent image carrier and can form a thin layer of developer on the image developing roll, characterized in that said image developing roll is composed of a metallic roll having, a rough surface morphology formed by sand blast treatment at least in the developer carrying region of the roll surface, and that anodizing is applied at least to the part of the surface where the rough surface morphology is formed.
Further, the present invention provides a developing device comprising at least an image developing roll which can carry developer on its peripheral surface and convey it a latent image carrier, characterized in that said image developing roll is composed of a metallic roll having a rough surface morphology formed by sand blast treatment at least in the developer carrying region of the roll surface, and that anodizing is applied at least to the part of the surface where the rough surface morphology is formed.
The developing device according to the present invention is further characterized in that said developer is composed of developer particles and external additives with a predetermined hardness adhering to the former, and that the hardness of the surface of said metallic roll is set to be lower than the hardness of said external additives.
The developing device according to the present invention is further characterized in that said developer is non-magnetic monocomponent toner.
The developing device according to the present invention is further characterized in that the particle diameters of said external additives are set to be smaller than the diameters of minute holes existing in the oxidation layer formed by said anodizing.
The developing device according to the present invention is further characterized in that the liberation ratio of said external additives is set to be 2% or more.
The developing device according to the present invention is further characterized in that the sphericity of said developer particles is set at 0.9-1 expressed in Wadell""s practical sphericity value.
The developing device according to the present invention is further characterized in that the peripheral speed of said image developing roll is set to be higher than the peripheral speed of said latent image carrier.
In the developing device having the aforementioned constitution according to the present invention, at least developer carrying region of the surface of the metallic roll composing the image developing roll has a clear surface morphology formed by the sand blast treatment, in which sharp ridges bound adjacent recesses. This roughened region of the metallic roll surface is subsequently subjected to anodizing, in which an oxidation layer is formed on the roughened surface. In this respect, most of the electrolysis reaction occurs penetrating into the metallic roll substance, so that the thickness of the oxidation layer formed at the surface is very thin. Therefore, the formation of the oxidation layer at the metallic roll surface hardly influences the surface morphology, such that the rough surface morphology formed in the sand blast treatment is mostly preserved after anodizing. Thus, the ridges bounding the adjacent recesses can be clearly formed, and the roll can reliably exhibit the developer conveying property because of the edge effect of the roughened surface.
Preserving of sharp ridges in the surface morphology formed by the sand blast treatment also increases the contact area between the image developing roll and developer particles. By this, the friction between the image developing roll surface and developing particles is sufficiently promoted such that the developer particles are effectively friction charged. Thus, the developer charging property of the image developing roll can be improved. Particularly, in a developing device employing a non-contact developing system in which the image developing roll is not in contact with the latent image carrier, the improvement in developer conveying property and in developer charging property brings about a decrease of toner scattering.
In addition, the oxidation layer formed by anodizing hardens the surface of the image developing roll, improving both abrasion resistance and mechanical strength of the image developing roll. In this respect, the hardness of the image developing roll surface can be further increased by performing the treatment slowly in an electrolysis solution kept at relatively low temperature.
This oxidation layer has a cellular structure consisting of a thick porous layer with many minute holes and a dense and very thin active layer. In this respect, the active layer has a certain electric resistance such that a metallic roll having a relatively low electric resistance comes to have a predetermined electric resistance, because of the resistant layer formed at the roll surface by anodizing. An uniform electric resistance can be obtained over the anodic-oxidized part of the metallic roll surface, since the anodic oxidation occurs uniformly over the treated surface of the metallic roll. Thus, an image developing roll can be formed easily at a low cost, from a metallic roll having an uniform electric resistance of a predetermined value, requiring no special electrically resistant material for the manufacture of the image developing roll.
The image developing roll having an uniform electric resistance of a predetermined value can prevent the injection of excess charge from developing bias to developer. This effect is particularly important in a developing device employing a contact developing system, in which a large pressure exerted by developer particles nipped between a latent image carrier and an image developing roll promotes the injection of excess charge to developer.
Further, by setting the hardness of the metallic roll surface to be lower than the hardness of external additives of toner according to the present invention, an effect is obtained that the rough surface of the image developing roll is scraped or broken little by little by the abrasion with the external additives of toner, the effect suppressing the adhering of toner onto the image developing roll surface, thereby effectively preventing the occurrence of filming. This effect is particularly important in a developing device employing a contact developing system in which a regulating member is in contact with an image developing roll, and the toner is prone to adhere at the location where the regulating member touches the image developing roll surface. This gradual breaking or scraping of the rough surface of the image developing roll by external additives of toner is however so little by little that the abrasion resistance of the image developing roll is maintained, ensuring an uniform toner conveyance and an appropriate toner charging.
In addition, a gradual breaking of the ridges in the rough surface morphology of the image developing roll leads to the formation of new ridges in the surface morphology.
The oxidation layer on a roll surface treated with anodic oxidation has many minute holes, which not only attract: foreign materials but make the roll surface susceptible to corrosion, resulting in deteriorated developing effect. Therefore, a sealing treatment is required in order to deactivate the holes to improve the stability of the roll surface against environment. On the other hand, a non-magnetic monocomponent toner used in the present invention is composed of toner mother particles and external additives adhering to the former. In this respect, not entire external additives are attached to the mother particles but a part of them exist liberated from the mother particles. Since according to the present invention, the particle diameters of external additives are set to be smaller than the diameters of the minute holes, the external additives can be embedded in the minute holes during toner is carried and conveyed on the image developing roll surface, thus sealing the many minute holes with external additives. In this manner, the sealing of the minute holes can be achieved automatically by means of the liberated external additives of toner during the toner conveying.
Therefore, a special treatment for sealing the many minute holes are not required, or even if the special sealing treatment is required, the treatment can be simplified because of the aid from the sealing effect of the liberated external additives 3b. Particularly, by setting the liberation ratio of the external additives to be 2% or more, many minute holes are effectively sealed with liberated external additives, while at the same time suppressing the occurrence of filming because of intervention of liberated external additives between the toner particles and the image developing roll surface or the latent image carrier surface.
Further, by setting the sphericity of the developer particles at a range of 0.9-1 expressed in Wadell""s practical sphericity according to the present invention, the developer particles can rotate and slide more freely, ensuring an effective recharging of the developer particles. As a result, developer particles existing in the non-image portion of the latent image carrier can be recovered more reliably, while preventing a scattering of image in the image portion of the latent image carrier, thus ensuring a high fidelity visualization of high definition image on the latent image carrier.
Further, by setting the peripheral speed of the image developing roll to be higher than the peripheral speed of the latent image carrier according to the present invention, developer particles can be effectively recharged by the rotating and sliding actions of the developer particles caused by the speed difference between the image developing roll and the latent image carrier in the contact region of the two rolls. In this manner, the charging level of defectively charged developer particles can be increased. As a result, developer particles existing in the non-image portion of the latent image carrier can be recovered more reliably, and the developer particles in the image portion of the latent image carrier can adhere to the right locations, preventing so-called scattering of image, which is caused by deviation of the adhering locations of developer particles.
Thus, in a developing device according to the present invention, an image developing roll can perform the foregoing three functions more reliably. As a result, a developing device according to the present invention can yield high quality images without image defects such as mottling for a long period.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.